Silica | CeramicSearch.com |
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QUARTZ, FLINT, SILICA
Chemistry
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NotesThe term 'siliaca' can be misleading. It is important to understand the difference between 'silica mineral', 'silicates', and 'silica glass'. Quartz is the best example of a natural mineral that is almost pure silicon dioxide (it is the most abundant mineral on planet earth). Other ceramic minerals like feldspar and clay contain some 'free silica' (accessory guartz). However these also usually contain 'silicates', that is, SiO2 chemically combined with other oxides to form crystalline minerals. Other silica-containing rocks and minerals are andalusite, barite, beach sand, bentonite, calcite, diatomaceous earth, kaolin, limestone, mica, pyrophyllite, talc, tripoli, rutile, wollastonite, zeolite, zirconium sand, vermiculite, granite, and sandstone. Silica is also available as a silicate glass (in frits).Pure silica minerals (like quartz) have high melting points. In ceramic bodies and glazes other oxides are added to complement it, they form silicates with it or occupy the network between particles of quartz. In the latter case silica is considered a 'filler' (e.g. porcelain clay bodies). It is interesting that some special purpose (and expensive) clay bodies replace the silica filler with calcined alumina, this greatly increases body strength and reduces thermal expansion. Individual particles of quartz have a high thermal expansion (and associated contraction) and significantly change their volume as they pass up and down through 'inversion' temperature points during firing. This can cause a form of body cracking called 'dunting'. It occurs as microcracks radiate out from each microscopic particle of quartz and propagate into larger cracks. High quartz bodies are usually unsuitable for ovenware and ware that must tolerate sudden temperature changes. However this behavior is advantageous to glaze fit since it puts the 'squeeze' on the glaze to prevent crazing. At the same time silica in glazes tends to dissolve and form low expansion silicates that reduce glaze expansion and also prevent crazing. In both cases, silica powder of small grain size is advantageous. High temperature bodies tend to have up to 30% silica whereas low fire ones have much less or none (because of its refractory nature). High temperature glazes can have 40% or more silica at times, if enough flux is available to react and form silicates. Understanding that quartz mineral and silica glass have vastly different physical properties is often the beginnings of understand the relationship between the mineralogy of the materials we use and their chemistry. Fused silica glass, for example, in one of the lowest thermal expansion materials available. Some industries, for example, use fused silica slabs weighing more than a ton as valves in large pipes where temperatures are not only high but suddenly change, yet these slabs do not crack. Quartz, on the other hand, is one of the least thermal expansion tolerant minerals. Yet both are SiO2!
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CAS: 14808-60-7 Silica: ACGIH TLV & OSHA PEL: 0.1 mg/cubic meter 8 hr. TWA, Respirable Dust. By comparison iron oxide is considered a safe-to-use material at a TLV of 5.0, kaolin is 2.0, barium carbonate is 0.5, quartz is 0.1-0.05.
Silica is contained in many ceramic minerals (e.g. feldspar), including its pure form of flint. Inhalation over long periods will cause silicosis where fibrosis of the lungs causes shortness of breath and can lead to death in severe cases. Over exposure to silica weakens the body's defense mechanisms.
Because of the abundance of silica, potential hazards are widespread. NIOSH in the US has estimated that 3.2 million workers in the USA are exposed to silica dust. Industries affected are quarrying, mining, steel, iron, metal foundries, abrasive blasting, construction, glass and ceramics, paint and pigments, granite and stone industries.
The primary health risk is the inhalation of "respirable" particles smaller than 10 micrometers (about 1/2500th of an inch). Generally, the smaller the particles are, the greater hazard and potential injury to the lungs. Dust particles larger than these are not capable of penetrating the defense mechanisms of the lung. Prolonged exposure may cause delayed chronic lung disease-silicosis. Chronic silicosis may take many years of exposure to develop, but with acute exposure rapid development can occur. In latter stages of silicosis, known as complicated or conglomerate silicosis, lung function may be reduced, resulting in symptoms of shortness of breath.
Quartz rock can be calcined so that it breaks down and grinds much easier. When this is done the material poses a much greater health risk.
The International Agency for Research on Cancer (IARC) has determined from a review of human epidemiology studies that there is limited evidence for the carcinogenicity of crystalline silica.
In the USA, occupational exposure is regulated by OSHA and under the laws of some states. The mining industry is regulated by the Mine Safety and Health Administration (MSHA). In 1992 the permissible exposure limit based on an 8-hour time weighted average concentration of respirable silica is .1 milligrams of quartz per cubic meter of air. OSHA and ASTM agree on this figure.
Studies have been somewhat mixed in results, even where when confounding exposures of other carcinogens including radon and tobacco are involved.
Findings on cancer risks associated with silica dust have made it necessary in some states that material safety data sheets reference the findings of IARC.
Silica can be used and handled safely with appropriate work practices and the avoidance of prolonged exposure. Dust concentrations past the limit are invisible to the naked eye, so employers must monitor workers using approved sampling methods.
See Crystalline Silica Q&A from National Industrial Sand Association, 900 Spring Street, Silver Spring, MD 20910.
See The American Ceramic Society Bulletin Aug 2000 Page 60 for information on the Crystalline Silica Work Group of ACerS.
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